Variation and inheritance

Question 1

What are the causes of continuous variation?

Answer 1

• Genetic and environmental.
• Polygenes

Question 2

What are the causes of discontinuous variation?

Answer 2

• Mostly genetic (one or two genes)

Question 3

What is codominance?

Answer 3

• When two different alleles occur for a gene - both of which are equally dominant.
• As a result both are expressed in the phenotype.

Question 4

What is an example of a gene with multiple alleles?

Answer 4

• Blood group (immunoglobulin gene - Gene I)

Question 5

What is sex linkage?

Answer 5

• Where a characteristic is determined by genes on the sex chromosomes.

Question 6

Why are characteristics caused by recessive alleles more common in males?

Answer 6

• The Y chromosome is much smaller than the X chromosome.
• There are a number of genes in the X chromosome that males have only one copy of.
• This means that any charactersitic caused by a recessive allele on this section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males.
• More commonly masked in females as they can have a dominant allele present in their cells.

Question 7

What is an example of a sex-linked genetic disorder?

Answer 7

• Haemophilia.
• Absence of a protein blood-clotting factor (often factor VIII).

Question 8

What is a dihybrid cross?

Answer 8

• Used to show inheritence of two different characteristics, caused by two genes.
• May be located on different pairs of homologous chromosomes.

Question 9

Why may the ratios observed in dihybrid crosses differ from expected?

Answer 9

• Due to linkage.

Question 10

What is autosomal linkage?

Answer 10

• When two genes, located closely together on the same chromosome, are inherited as one unit.
• Unless they are separated by a chiasmata during crossing over, they will not be independently assorted.

Question 11

What are recombinant offspring?

Answer 11

• They have different combinations of alleles than either parent. The closer the genes are together on a chromosome, the less likely they are to be separated.

Question 12

How can you calculate recombination frequency?

Answer 12

number of recombinant offspring / total number of offspring

Question 13

What does a recombination of less than 50% indicate?

Answer 13

• There is gene linkage.
• The random process of independent assortment has been hindered.

Question 14

What is epistasis?

Answer 14

• The interaction of genes at different loci.
• Gene regulation is a form of epistasis: regulatory genes control the activity of structural genes.
• Masking of a gene by another allele

Question 15

What is the difference between dominant and recessive epistasis?

Answer 15

• Dominant epistasis: only one allele needs to be inherited to masl the expression of a gene at another locus.
• Recessive epistasis: two copies of a recessive allele must be inherited to mask the expression of a gene at another locus.

Question 16

What does the Hardy-Weinberg principle assume?

Answer 16

• Diploid organisms
• Large & isolated population
• Random mating
• No mutations
• No selection pressures

Question 17

What does mutation affect evolution?

Answer 17

• Mutation is necessary for the existence of different alleles in the first place, new alleles = genetic variation.

Question 18

How does sexual selection affect evolution?

Answer 18

• Sexual selection leads to an increase in frequency of alleles which code for characteristics that improve mating success.

Question 19

How does gene flow affect evolution?

Answer 19

• Gene flow, movement of alleles between populations. Immigration and emigration result in changes of allele frequency within a population.

Question 20

How does genetic drift affect evolution?

Answer 20

• Genetic drift occurs in small populations. Change in alleke frequency due to random nature of mutation. A new allele will have a greater impact in a smaller population.

Question 21

How does natural selection affect evolution?

Answer 21

• Natural selection leads to an increase in the number of individuals that have characteristics that improve their chances of survival. These individuals will have higher reproduction.

Question 22

What are density-dependent factors?

Answer 22

• Affected by population size.
• Competition, predation, parasitism, communicable disease.

Question 23

What are density-independent factors?

Answer 23

• Unaffected by population size.
• Climate change, natural disasters, seasonal change, human activities.

Question 24

What is the founder effect?

Answer 24

• When a small population arises due to the establishment of new colonies by a few isolated individuals, leading to the founder effect.
• Extreme example of genetic drift.
• New, small populations have much smaller gene pools & display less genetic variation.
• Previously rare alleles can massively increase in frequency if carried to the new population.

Question 25

What is a genetic bottleneck?

Answer 25

- A large reduction in population size due to a catastrophic event, last multiple generations. - Gene pool and genetic diversity greatly reduced.

Question 26

What is stabilising selection?

Answer 26

- Individuals with average characteristics are more likely to survive and reproduce than those at the extremes. - Positive selection of norms, extremes selected against (negative selection)

Question 27

What is directional selection?

Answer 27

- A change in the environment occurs, meanning the most common phenotype is no longer the most advantageous. - Organisms which are less common and have more extreme phenotypes are positively selected, evolution occurs. - The whole bell curve shifts to one direction.

Question 28

What is disruptive/diversifying selection?

Answer 28

- The extremes are selected for and the norm selected against. - E.g Darwin's finches

Question 29

What is allopatric speciation?

Answer 29

- More common form of speciation, happens when some members of a population are geographically isolated. - Each populations' environment is different, so different selection pressures will result in different physical adaptations.

Question 30

What is sympatric speciation?

Answer 30

- Occurs within popualtions that share the same habitat. - Less frequently occurs. - Can occur when members of 2 different species interbreed and form fertile offspring - often happens in plants. The hybrid has a different number of chromosomes to either parent. Can no longer interbreed. - Reproductive isolation.

Question 31

What are the problems caused by inbreeding?

Answer 31

- Limited gene pool = decreased genetic diversity. - This limits the chance of evolution occuring and limits their ability to adapt. - Closely related organisms are genetically similar and are likely to have the same recessive alleles. - Inheritance of two recessive alleles can cause certain genetic disorders.